One type of a storage device has a device sleep function. The device sleep function is a function of entering a low power consumption state (hereinafter, device sleep state) in response to a trigger signal from a host. Before the storage device enters the device sleep state, data stored in volatile memory such as a DRAM or an SRAM is transferred to nonvolatile memory such as a NAND flash memory, and after that, power supply to the storage device is cut off.
However, each time the storage device enters the device sleep state, the data in the volatile memory is transferred to the nonvolatile memory. As the writing in the nonvolatile memory is carried out each time, the performance of the nonvolatile memory becomes lower.